Hi!

Thanks a lot for this library.

Looking at the code I'm suspicious that there is a race in Close():

Is it possible that two concurrent goroutines running Close() could both pass the check function w.isClosed() and then both call close(w.done)? That would result in two closes of the channel and a panic.

One solution would be to lock the w.mu at least around the w.IsClosed and close(w.done) calls.